This disclosure relates generally to the field of semiconductor fabrication, and more specifically to ultraviolet (UV) curing of nitride film to enhance a tensile stress in the nitride film.
Manipulating stress is an effective way of improving the carrier mobility in a complementary metal oxide semiconductor (CMOS) field effect transistor (FET). When stress is applied to the channel of a FET, the mobility of carriers, and as a consequence, the transconductance and the on-current of the FET, are altered from their original values for an unstressed channel FET. This is because the applied stress and the resulting strain on the semiconductor structure within the channel affects the band gap structure (i.e., breaks the degeneracy of the band structure) and changes the effective mass of the carriers. The effect of the stress depends on the crystallographic orientation of the plane of the channel, the direction of the channel within the crystallographic orientation, and the direction of the applied stress.
The effect of uniaxial stress, i.e., a stress applied along one crystallographic orientation, on the performance of semiconductor devices, especially on the performance of a FET device built on a silicon substrate, has been extensively studied in the semiconductor industry. For a p-type FET (PFET) with a silicon channel, the mobility of carriers (which are holes in the case of a PFET) in the channel increases under uniaxial compressive stress along the direction of the channel, i.e., the direction of the movement of holes or the direction connecting the drain to the source. Conversely, for an n-type FET (NFET) device with a silicon channel, the mobility of carriers (which are electrons in the case of an NFET) increases under uniaxial tensile stress along the direction of the channel, i.e., the direction of the movement of electrons or the direction connecting the drain to the source.
The stress in an FET channel may be induced by a stressed nitride film that is formed over the FET device. A nitride film formed over a PFET device may have a compressive stress, and a nitride film formed over an NFET device may have a tensile stress. UV curing of a tensile stressed nitride film may enhance the tensile stress in the nitride film by reconfiguring silicon-hydrogen/nitrogen-hydrogen bonds present in the nitride film. UV curing of nitride is performed by exposing the nitride to UV radiation, which has a wavelength in a range from about 10 nanometers (nm) to about 400 nm. The enhanced stress in the nitride film induces a corresponding enhanced stress in a channel of the FET over which the UV cured nitride film is located, increasing the carrier mobility in the FET channel.